Sidestick Priority Logic

Pilot Controls established the most counter-intuitive fact in the chapter: the two sidesticks are springloaded to neutral, not mechanically linked, and take no feedback from the surfaces. Move yours and the other pilot's stick does not move; pull hard and the force in your hand is the artificial-feel spring, not the aerodynamic load. That single design choice creates a question a conventional yoke aircraft never has to answer: two pilots, two sticks, both able to move at once — whose order does the aircraft fly?

The answer is the sidestick priority logic. With no priority taken, the two stick orders are algebraically summed and limited to one full stick. Either pilot can take over with the priority (takeover) pushbutton, deactivating the other stick. The whole sequence is made audible and visible — a "DUAL INPUT" aural, "PRIORITY LEFT/RIGHT" callouts, the glareshield SIDE STICK PRIORITY arrows (green and red), and the white sidestick crosses on the PFD. This article is the CRM-flavoured capstone of the law and protection group: it is as much about what the crew must say as about what the computers do.

[!warning]- The green light does not mean "I have priority", and the system never decides who is right.

Two myths to clear before anything else. First, you cannot feel the other pilot fly — the sticks are uncoupled and give no feedback, so the lights, the aural, and the PFD crosses are the only substitute for the conventional "I can see you pulling". Second, when both sticks move the computers do not arbitrate who is correct — they add the two orders algebraically and shout "DUAL INPUT". Resolving a dual input is a crew task ("I have control"), not a computer task. Carry the yoke model — sticks that back-drive, a system that picks a winner — into this chapter and every line below will read wrong.

1. Why a priority logic exists — the uncoupled stick

Start from the physics, because every warning that follows is a consequence of it. Per FCOM DSC-27-20-30:

The sidesticks are springloaded to neutral. They are not mechanically linked, and do not receive feedback from the flight control surfaces. When the autopilot is engaged, the sidesticks are locked in the neutral position.

On a yoke aircraft the two control columns are mechanically tied: if the other pilot hauls back, your column comes back too, and you feel it instantly. The A330 deliberately gives that up. The benefit — a clear view of the panel, a sliding table, room to fly with the wrist — comes at one cost: the natural, mechanical "I can see and feel what you are doing" is gone. The priority logic, the lights, the aural warnings, and the PFD crosses exist to rebuild that lost awareness electronically.

Each stick carries a pushbutton with a dual role, and that pushbutton is the single control behind everything in this article. Per FCOM DSC-27-20-30:

Each sidestick has two controls: ‐ The Radio Push To Talk switch ‐ The sidestick pb, for autopilot disengagement, or sidestick priority.

So the sidestick pb is both the autopilot-disconnect and the priority (takeover) button — the context decides which job it does. With the autopilot engaged it is the AP disconnect; with the AP out and both pilots flying, it is the takeover button. (The AP-engaged neutral lock and the 5 daN pitch / 3.5 daN roll override force that frees the stick are detailed in Pilot Controls; here the stick is assumed unlocked and in manual flight.)

[!warning]- You cannot sense the other pilot's input through your hand — there is no tactile channel at all.

The instinctive prediction is "if the Captain is holding full right stick, I will feel it or see my stick deflect." Per FCOM the sticks are not mechanically linked and do not receive feedback — so the other stick sits dead still and your hand feels only its own spring. Your only cues that the other pilot is flying are the PFD sidestick cross, the aircraft's attitude change, and — if you are both on the sticks — the green lights and the "DUAL INPUT" aural. This is exactly why Airbus writes "announce your inputs" into SOP (§8): the airframe will not announce them for you.

2. Architecture — two sticks, one pushbutton each, one set of lights

Three pieces implement the whole logic:

The decision logic is best read as a state machine. The judgement rules below are taken sentence-by-sentence from FCOM DSC-27-20-30; the AMM names the function Priority Logics but the operative rules live in the FCOM text, so this is a logic diagram built from that text, not a traced figure.

                  ┌──────────────────────────────────────────┐
                  │ STATE 0 — single pilot (normal)            │
                  │ one stick deflected · the other at neutral │
                  │ lights off · no aural · PFD crosses (gnd)  │
                  └─────────┬───────────────────────┬─────────┘
       both sticks off      │                       │ either pilot
       neutral              ▼                       ▼ presses the pb
  ┌──────────────────────────────┐    ┌──────────────────────────────────┐
  │ STATE A — DUAL INPUT          │    │ STATE B — PRIORITY (takeover)     │
  │ two orders ALGEBRAICALLY      │    │ presser takes FULL control        │
  │ SUMMED, limited to one full   │    │ other stick is cut out            │
  │ stick                         │    │ presser GREEN (if other stick     │
  │ both GREEN arrows FLASH       │    │   not at neutral) · loser RED     │
  │ "DUAL INPUT" every 5 s        │    │ "PRIORITY LEFT/RIGHT" once        │
  └──────────┬───────────────────┘    └────────┬──────────────────┬──────┘
             │ a pb is pressed                  │ keep pb          │ other stick
             └─────────────────────────────────►│ pressed ≥ 40 s   │ to neutral
                                                 ▼                  ▼
                          ┌──────────────────────────────────┐  GREEN off
                          │ LATCH — other stick permanently   │  (priority is
                          │ deactivated; release the pb and   │   NOT lost)
                          │ it stays off, until ANY pilot     │
                          │ presses a pb to release           │
                          └──────────────────────────────────┘

Read three things off it:

1. The default (STATE A) is "sum and limit" — nobody is privileged. The system treats the two sticks as equals and only breaks that symmetry when a pilot acts on the pushbutton.
2. A press gives short-term priority (STATE B); a 40-second hold gives a permanent LATCH. They are two different things (§5).
3. The "lights off / unlatch" paths are several and easily confused — the green and red arrows extinguish for different reasons (§6).

3. No priority taken — the algebraic summation

In the default state the system does not pick the larger input, nor a favoured side. It adds the two orders with their signs. Per FCOM DSC-27-20-30:

At all times, only one flight crewmember should fly the aircraft. However, if both flight crewmembers use their sidesticks simultaneously, their orders are algebraically added. The flight control laws limit the combined order to the equivalent of the full deflection of one sidestick.

Two parts, and the sign is where the danger hides:

• Same direction — both pilots pull aft: the orders add, but the law caps the sum at one full stick. So two pilots pulling together is not double authority; at most it equals one pilot at the stop. There is no "double over-control" failure mode.
• Opposite direction — one pulls, one pushes: the algebraic sum partially cancels. This is the dangerous case: the combined order can fall near zero, the aircraft barely responds, and each pilot can believe they are in control. (That the wrong direction — not too much force — is the real trap is reasoning drawn from "algebraically added"; FCOM states the summation, not this consequence.)

[!warning]- The hazard of a dual input is "too little", not "too much".

The instinct is that two pilots both shoving the stick will over-control the aircraft. The opposite is the trap. Same-direction inputs are limited to one full stick, so they cannot over-stress anything the system would not allow one pilot to command. The genuine hazard is opposite inputs that algebraically cancel: the aircraft sits roughly still while both pilots are convinced they are flying it. The fault is direction, not magnitude — which is exactly why the cure is verbal ("I have control"), not muscular.

Think of the two sticks as two people writing into the same box on one order form, then a cap stamped over the total. The system does not listen to whoever is louder — it adds the two signed numbers and refuses to let the total exceed "the most one person could write". The catastrophe is not "+max and +max"; it is "+max and −max = 0" — the form reads nothing ordered, while both believe they placed a full order.

4. DUAL INPUT — the aural and visual warning

The instant both sticks leave neutral, the system announces it with light and voice. Per FCOM DSC-27-20-30:

In this case the two green SIDE STICK PRIORITY lights on the glareshield come on and "DUAL INPUT" voice message is activated.

The rhythm and the stop condition of the aural are pinned down by the warning table. Per FCOM DSC-31-10:

"DUAL INPUT" (synthetic voice) | Both sidesticks are moved simultaneously | Every 5 s | One sidestick deactivated

So the call repeats every 5 seconds for as long as both sticks are moved, and stops only when one sidestick is deactivated (a takeover) — or, equally, when a pilot releases to neutral and the dual input ends. The AMM adds the dynamic of the lights: in a dual input the green arrows do not merely come on, they flash, and only stop flashing once the priority logic resolves. Per AMM 27-90-00:

When the two side sticks are deflected simultaneously the two green indications flash and an aural warning sounds. When one priority pushbutton switch is pushed, the indications stop flashing and the priority logic is achieved.

What this means in the cockpit: a dual input is not an error the system fixes for you. It does not decide who is right; it simply shouts that you are both flying and forces you to resolve it with words. The "DUAL INPUT" call repeats every 5 s until either one pilot returns to neutral or one pilot takes priority with the pushbutton.

5. Taking over — the priority pushbutton and the 40-second latch

To remove the other pilot from the loop entirely, press and hold your own pushbutton. Per FCOM DSC-27-20-30:

A flight crewmember can deactivate the other sidestick and take full control, by pressing and keeping pressed the sidestick pb. To deactivate the other sidestick, the flight crewmember must press their sidestick pb for 40s. The other sidestick is permanently deactivated, until any flight crewmember presses their sidestick pb. If both flight crewmembers press their sidestick pb, the last pilot to press gets the priority.

Three points that are routinely confused:

• A press is not a 40-second hold. A short press gives priority only while you keep it held — release it (before the latch) and the other stick recovers. Only a continuous hold of ≥ 40 s latches the other stick permanently: release after that and the other stick stays deactivated until someone presses a pb. The intent is to cover incapacitation or a jammed stick — you cannot fly with a thumb pinned to the button forever, so the 40-second latch frees your hand.
• The last to press wins. If both pilots press, the system does not freeze on the first presser — it follows the last press. The logic guarantees "if I decide to take over now, pressing will get it to me"; there is no deadlock where an earlier press locks you out.
• Any pilot unlatches. After a latch, the release is not restricted to the pilot who lost authority — either pilot's press cancels the latch and restores both sticks.

The takeover is announced once. Unlike the every-5-seconds "DUAL INPUT", the priority call is a one-shot verdict. Per FCOM DSC-27-20-30:

Sidestick priority audio: a "PRIORITY LEFT" or "PRIORITY RIGHT" audio voice message is given each time priority is taken.

The warning table gives its timing — a single ~1-second call, not a repeating one. Per FCOM DSC-31-10:

"PRIORITY LEFT" "PRIORITY RIGHT" (synthetic voice) | AP TAKEOVER pb | 1 s | NIL

A useful pairing to memorise: "DUAL INPUT" every 5 s is the nag — the problem is unresolved; "PRIORITY LEFT/RIGHT" once is the gavel — the verdict is in. One keeps repeating because nobody has fixed the conflict; the other sounds once because a decision has been made.

The clearest real-world use of the 40-second latch is incapacitation, and the FCTM ties the two together. Per FCTM (Flight Crew Incapacitation): if the incapacitated pilot interferes with the controls, press the sidestick pb for 40 seconds, and note:

The time required of 40 s includes the time necessary for AP deactivation (if AP engaged) and the time for offside sidestick deactivation.

So the 40 seconds is not pure margin — it bundles the AP disconnect (the same pb does that job) and the offside-stick deactivation into one continuous hold. Hold it for the full count and let go; the other stick stays out, and you fly single-pilot.

6. The priority lights — exactly who, exactly when they go out

This is the most-misremembered part of the topic, so the glareshield wording has to be set out in full. The red arrow always points at the pilot losing authority. Per FCOM DSC-27-20-30:

Arrow red lt: ‐ Illuminates in front of the flight crew losing authority. ‐ Extinguishes if flight crew has recovered their authority, ie: • If the other take-over pushbutton is released prior priority condition is latched, or • If flight crew has used their take-over pushbutton to cancel a latched priority situation.

The green arrow points at the pilot who has taken priority, or lights on both sides during a dual input. Per FCOM DSC-27-20-30:

CAPT-F/O green lt: ‐ Illuminates in front of the flight crew who has taken priority by pressing the takeover push button if the opposite stick is not at neutral. or ‐ CAPT and F/O light illuminate in case of simultaneous input on both sidesticks. ‐ Extinguishes when the opposite stick is returned to the neutral position.

The AMM compresses the pair into one statement and adds the detail that the EWD message appears only for the take-off configuration warning. Per AMM 27-90-00:

‐ Green indication of SIDE STICK PRIORITY annunciator comes on in front of the pilot who has priority when the side stick that has no priority is not at zero. ‐ Red indication of SIDE STICK PRIORITY annunciator comes on in front of the pilot who has lost priority. ‐ A message is shown on the EWD (for Take Off CONF. warning only). ‐ An aural warning sounds.

Distil the two arrows into their lit-and-extinguish rules:

[!warning]- The green arrow goes out when the other stick returns to neutral — that is not "you lost priority".

The intuitive reading is "green = I have priority, so it stays lit to remind me I'm flying." It does not. The green arrow means "the no-priority stick is off neutral" — i.e. there is still a potential conflict to flag. The moment the other pilot relaxes their stick to neutral, the conflict is gone and your green arrow extinguishes — even though you still hold priority. A dark green light therefore does not mean you have been deactivated; it usually just means the other pilot finally let go. This is a classic exam trap.

7. PFD indication and the ground-takeover trap

Beyond the lights and aural, "who is moving the stick" is drawn on the PFD — but only on the ground. Per FCOM DSC-27-20-30:

On ground, after first engine start, side stick position indications appear white on both PFDs. The indication disappears when the Nose Landing Gear is fully extended.

On the ground the nose gear strut is compressed and the crosses are shown; at lift-off the nose gear extends to its fully-extended (down) position, the system reads "airborne", and the crosses vanish. The PFD cross is in effect a "still on the ground" visual flag, used with the pre-flight flight-control check (the PM calling "full up/down/neutral" — see Controls and Indications).

If a stick was latched out on the ground and never released, the system catches it on take-off. Per FCOM DSC-27-20-30:

If one sidestick was deactivated on ground, the CONFIG L(R) SIDESTICK FAULT alert is triggered at takeoff power application, or during the TO CONFIG test.

The fix is a single action — restore the deactivated stick. Per FCOM PRO-ABN-CONFIG, the procedure for CONFIG L(R) SIDESTICK FAULT (BY TAKE OVER) is:

L (R) TAKE OVER .................... PRESS The affected stick becomes operative.

A near-identical alert with a completely different cause must be held apart from this one. The CONFIG alert above is a deliberate-takeover left latched — human, recoverable. But FCOM also carries F/CTL L(R) SIDESTICK FAULT, which is a real transducer failure, nothing to do with a stray latch. Per FCOM PRO-ABN-F_CTL, its triggering condition is:

This alert triggers when the transducers on pitch or roll axis are failed on one sidestick.

Its handling line is just acknowledgement — there is no on-side action that recovers it, and the stick is declared inoperative. Per FCOM PRO-ABN-F_CTL:

Crew awareness.

with STATUS / INOP SYS / L(R) SIDESTICK. Read the prefix to tell them apart:

[!warning]- SIDESTICK FAULT on the screen — read the prefix before you touch anything.

CONFIG (prefix) means you or the ground crew left a stick latched out: press L(R) TAKE OVER and it clears. F/CTL (prefix) means a transducer has genuinely failed: the pushbutton does nothing, that stick is INOP for the sector, and you fly on the other one — which the uncoupled, redundantly-split design (the four potentiometer groups of Pilot Controls) is built to allow. Some operators' MEL treats both as no-dispatch / run-the-ECAM, but the root cause — human versus hardware — is opposite.

8. CRM — the logic is a backstop, not a substitute

The priority logic is the machine's safety net; it does not replace crew coordination. The FCTM is blunt about the drill. Per FCTM (Use of Sidestick):

Only one flight crew flies at a time. If the PM wants to act on the sidestick, the PM must: ‐ Clearly announce "I have control" ‐ Press and maintain the sidestick pushbutton, in order to get full control of the Fly-By-Wire system. The flight crew should keep in mind that sidestick inputs are algebraically added. Therefore dual inputs must be avoided, and will trigger aural and visual alerts. Either flight crew can make an input on their sidestick at any time. Either flight crew can deactivate the other flight crew's sidestick by pressing on their sidestick pushbutton.

The FCTM also names the trade the side-stick design makes — it sacrifices the natural "follow-through" awareness to gain real benefits in emergencies. Per FCTM (Fly-By-Wire), among the main operational benefits of the side-mounted stick:

‐ It is adapted for emergency situations (e.g. incapacitation, stick jamming, control failures)

The discipline that follows: call first, button second. The standard takeover is to announce "I have control" and then press and hold the pushbutton — the words handle "the human knows", the button handles "the machine knows". The 40-second latch is precisely the tool the FCTM keeps for those emergency cases — incapacitation, a jammed stick — where you cannot keep a thumb on the button and still fly.

Self-test

[!note]- Q1. Both pilots go to full stick — one pulls, one pushes. What does the aircraft do, and why is this worse than both pulling together?

The two orders are algebraically added, and the flight control laws limit the combined order to one full stick deflection (per FCOM DSC-27-20-30). One-pull-one-push is opposite-direction input: the signed sum partially cancels, the combined order can fall near zero, and the aircraft barely responds while both pilots believe they are in control. It is worse than both-pulling because the same-direction case is merely capped (at most one full stick — no double over-control), whereas the opposite case is the hidden "looks like flying, nobody is flying" trap. The system flashes both green arrows and calls "DUAL INPUT" every 5 s, but it does not decide who is right — the crew must resolve it with "I have control" and the pushbutton.

[!note]- Q2. "DUAL INPUT" and "PRIORITY LEFT/RIGHT" — compare their trigger and their rhythm.

"DUAL INPUT" triggers when both sidesticks are moved simultaneously and repeats every 5 s until one stick is deactivated (per FCOM DSC-31-10) — it is the nag of an unresolved conflict. "PRIORITY LEFT/RIGHT" triggers when a takeover pushbutton is pressed and sounds once (~1 s) (per FCOM DSC-31-10 and DSC-27-20-30) — it is the gavel: left or right names the side that has just taken priority.

[!note]- Q3. How do you take permanent control? If both pilots press, who wins? How is a latch released?

Press and hold your own sidestick pb for 40 s; the other stick is then permanently deactivated and stays out even after you release (per FCOM DSC-27-20-30). A short press gives priority only while held. If both pilots press, the last to press gets priority — the system does not freeze on the first, so "I want it now" always succeeds. After a latch, any pilot's press cancels it and restores both sticks.

[!note]- Q4. Where do the red and green arrows light, and what is the trap in the green arrow's extinguish condition?

The red arrow lights in front of the pilot losing authority; the green arrow lights in front of the pilot who took priority (while the other stick is off neutral), or on both sides during a dual input (per FCOM DSC-27-20-30). The trap: the green arrow extinguishes when the opposite stick returns to neutral — it signals "the no-priority stick is still off neutral", not "I hold priority". So your green light going dark usually means the other pilot simply let go; you have not lost priority. The red arrow extinguishes when you recover authority (the other pb released before the latch, or you cancel a latched priority with your own pb).

[!note]- Q5. When do the PFD sidestick crosses show and vanish? A stick is left latched out on the ground — what happens at take-off, and how does that alert differ from F/CTL L(R) SIDESTICK FAULT?

The white crosses appear on both PFDs on the ground, after first engine start, and disappear when the nose landing gear is fully extended (lift-off) — a "still on the ground" flag (per FCOM DSC-27-20-30). A stick latched out on the ground triggers CONFIG L(R) SIDESTICK FAULT at take-off power application or the TO CONFIG test; the fix is L(R) TAKE OVER … PRESS and the stick becomes operative (per FCOM PRO-ABN-CONFIG). That is a human mis-latch, recoverable. By contrast F/CTL L(R) SIDESTICK FAULT triggers when a pitch- or roll-axis transducer genuinely fails; the action is only Crew awareness, the stick is INOP, and the button does nothing (per FCOM PRO-ABN-F_CTL). Read the prefix: CONFIG = press the takeover; F/CTL = fly on the other stick.

[!note]- Q6. The priority logic exists — so why does the FCTM still insist on "I have control"?

Because the logic is the machine's backstop, not a substitute for crew coordination. Per FCTM, only one pilot flies at a time; to take the stick the PM must clearly announce "I have control" and press and maintain the pushbutton — words for the human, button for the machine. Dual inputs are algebraically added and must be avoided; they will trigger the aural and visual alerts. The uncoupled stick gives no tactile cue, so without the call-out the other pilot may not realise control has changed. Call first, button second.

Key takeaways

References

Per FCOM DSC-27-20-30 (Controls and Indicators — sidesticks springloaded/uncoupled/no feedback/AP neutral lock; the sidestick pb's dual role of AP disengagement or priority; sidestick priority logic — algebraic summation limited to one full stick, two green lights + "DUAL INPUT", takeover by press-and-hold, 40 s permanent deactivation, last-to-press wins, any pilot unlatches; glareshield red and green arrow lit/extinguish logic; "PRIORITY LEFT/RIGHT" sidestick priority audio; PFD sidestick crosses on ground; CONFIG L(R) SIDESTICK FAULT at take-off power / TO CONFIG test). Per FCOM DSC-31-10 (warning tables — "DUAL INPUT" trigger/every 5 s/suppression on one stick deactivated; "PRIORITY LEFT/RIGHT" on takeover pb, 1 s). Per FCOM PRO-ABN-CONFIG (CONFIG L(R) SIDESTICK FAULT (BY TAKE OVER) — L(R) TAKE OVER … PRESS, the affected stick becomes operative). Per FCOM PRO-ABN-F_CTL (F/CTL L(R) SIDESTICK FAULT — transducer pitch/roll axis failure, Crew awareness, INOP SYS L(R) SIDESTICK). Per AMM 27-90-00 (priority annunciator green/red logic; flashing green indications during simultaneous deflection and stop-flash on pushbutton press; EWD message for take-off configuration warning only). Per AMM 27-92-00 (sidestick function and artificial feel; higher-load neutral hold with the autopilot engaged while preserving override; single-failure split — no more than one of the four potentiometer groups per axis left free). Per FCTM (Use of Sidestick — "I have control" + press-and-maintain; dual inputs algebraically added and to be avoided; either pilot may input or deactivate at any time), FCTM (Fly-By-Wire — side-mounted stick benefits, adapted for emergency situations) and FCTM (Flight Crew Incapacitation — 40 s hold to deactivate the offside stick, which includes the AP-deactivation time). The opposite-direction cancellation consequence and the "call first, button second" framing are integrative synthesis built on the cited algebraic-summation and CRM statements, not single verbatim manual sentences.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.